Wesley Autrey, who jumped onto New York City subway tracks to save a man from an oncoming train. Captain Chesley Sullenberger of US Airways, who landed his plane and passengers safely on New York's Hudson River after birds knocked out both engines. Charles Ramsey, who helped free Amanda Berry from her kidnapper's Cleveland home. And most recently, Antoinette Tuff, who talked would-be Georgia school shooter Michael Brandon Hill into putting down his rifle so students could get out alive.

Actions like these might seem to be split-second snap judgments, but often they are a natural result of lives that have primed people for selflessness. Put another way: Heroes aren't born, they're made.

Expertise and training in helping others often spur people to act — rather than run or freeze — in a crisis. Even if someone hasn't faced a particular emergency before, extensive and even general preparation helps the brain act almost automatically.

Someone who keeps current with her CPR certification, for example, is more likely than your average bystander to be able to help a person who's not breathing. A pilot for more than 40 years, Sullenberger had flown many types of aircraft, including engineless gliders — experience that came in handy when he had to land a plane with two failed engines. When Hill barged into McNair Discovery Learning Academy on Aug. 20, toting an AK-47, Tuff relied on her special emergency response training as she tried to keep the situation under control.

Like those who have been trained to save lives or manage crises, individuals who are part of organizations promoting selflessness often have the making of heroes. Sullenberger was steeped in an aviation culture that stresses putting others first when danger threatens. And Autrey, who jumped on the tracks, was a veteran of the Navy, which has a "core values charter" that emphasizes doing the right thing, even if you face personal consequences.

In a 2006 survey of organizations with written codes of ethics, about two in three employees reported that the code affected their decision-making, and more than four in five said they applied their knowledge of the code regularly at work. Ethics codes and experience-based training typically complement one another: Codes provide a standard to strive for, and training gives people tools they need to meet that standard.

Other times, our difficult life experiences may help us identify with others' vulnerabilities in ways that awaken our generosity. Tuff showed Hill that she understood his despair and was able to persuade him to halt his deadly mission. "Don't feel bad, baby," she said to him, according to the tape of her 911 call. "My husband just left me after 33 years. … I've got a son that's multiple disabled." She told Hill that if she could recover from tough times, so could he. Science confirms the power of past difficulties to transform our outlook toward others.

How much of a role does personality play? Not necessarily a defining one. University of Winnipeg psychologist Jeremy Frimer reported that people who'd gotten a national Canadian award for bravery had personalities that were similar to those of people who hadn't gotten awards.

And despite their unselfish actions, heroes don't always have spotless backgrounds — sometimes far from it. Much has been made of Martin Luther King Jr.'s marital infidelity, Mahatma Gandhi's racist remarks and Charles Ramsey's reported history of domestic violence.

But these heroes' missteps don't detract from their selfless acts. In a way, it's reassuring that even heroes aren't perfect. Seeing their flaws gives the rest of us hope that regardless of our shortfalls, we, too, can do important work to benefit those around us.

Establishing good rapport with people does count, though. The more we believe we have things in common with others, the more motivated we may be to pitch in when they're in need. Kristen Monroe, a political scientist at the University of California at Irvine, reported that those who rescued people who were persecuted during World War II were more inclined to feel close to all fellow humans — not just those belonging to a particular race or group. They saw humanity almost as a large extended family, and that outlook helped compel them to act when seeing people in distress.

This mind-set might help explain why the people we consider heroes often reject the label: They see their selfless acts as part of their duty to other human beings. When CNN's Anderson Cooper asked Ramsey if he felt like a hero, he said: "No. No, no, no, no, bro. I'm a Christian and an American and I'm just like you."

After his daring subway rescue, Autrey said: "I don't feel like I did something spectacular; I just saw someone who needed help. I did what I felt was right."

Elizabeth Svoboda is the author of "What Makes a Hero?: The Surprising Science of Selflessness."

There is a reason why the air in Tampa Bay is filled with playoff talk. If Thursday night's 12-8 Bucs preseason win over the Jaguars is any indication, it's also going to be filled with footballs thrown by quarterback Jameis Winston.